How do some sites automatically open in Edge when tried to open in IE11?

You had asked, "Does anyone know how that works? Is it done via an internally compiled list of sites or something I can set up on my website?"

When a user goes to a site that is incompatible with Internet Explorer, they will be automatically redirected to Microsoft Edge. For more detailed information, you can refer to the Redirection from Internet Explorer to Microsoft Edge for compatibility with modern web sites.

Microsoft maintains a list of all sites that are known to be incompatible with Internet Explorer. You can view the site list here. If you want to add your site to the incompatible site list then you can refer to the Request an update to the IE compatibility list.

The MS Edge 87 or greater browser installs the BHO named IEToEdgeBHO that performs the redirection based on IE compatibility list from IE to Edge browser.

You can find this BHO at the location below on your machine.

C:\Program Files (x86)\Microsoft\Edge\Application\<Your_browser_version>\BHO

If you want to achieve something similar using the code in your own site then you can refer to the example on this answer. The test shows results for the Edge legacy browser but it can also work with the Edge (Chromium) browser.

Note: that the example code mentioned in the answer may break when the Edge browser is not installed on the machine. so you can try to modify the example to handle that kind of situation.

Helpful reference:

1. What “magic” causes “cnn.com” when typed in IE11 to automatically launch Edge (Chromium)?

Netflix specifically is on a list of websites that automatically open in Edge when they are accessed from Internet Explorer. The list is maintained by Microsoft.

If you do not want to go through the application process to get your website added to this list, you can resort to detecting which browser the user is on and handling the notice yourself.

Here is a decent list of current Internet Explorer user agents.